While organic electroluminescent (EL) devices have been known for over two decades, their performance limitations have represented a barrier to many desirable applications. In simplest form, an organic EL device is comprised of an anode for hole injection, a cathode for electron injection, and an organic medium sandwiched between these electrodes to support charge recombination that yields emission of light. These devices are also commonly referred to as organic light-emitting diodes, or OLEDs. Representative of earlier organic EL devices are Gurnee et al. U.S. Pat. No. 3,172,862, issued Mar. 9, 1965; Gurnee U.S. Pat. No. 3,173,050, issued Mar. 9, 1965; Dresner, “Double Injection Electroluminescence in Anthracene”, RCA Review, 30, 322, (1969); and Dresner U.S. Pat. No. 3,710,167, issued Jan. 9, 1973. The organic layers in these devices, usually composed of a polycyclic aromatic hydrocarbon, were very thick (much greater than 1 μm). Consequently, operating voltages were very high, often greater than 100V.
More recent organic EL devices include an organic EL element consisting of extremely thin layers (e.g. <1.0 μm) between the anode and the cathode. Herein, the term “organic EL element” encompasses the layers between the anode and cathode. Reducing the thickness lowered the resistance of the organic layers and has enabled devices that operate at much lower voltage. In a basic two-layer EL device structure, described first in U.S. Pat. No. 4,356,429, one organic layer of the EL element adjacent to the anode is specifically chosen to transport holes, and therefore is referred to as the hole-transporting layer, and the other organic layer is specifically chosen to transport electrons and is referred to as the electron-transporting layer. Recombination of the injected holes and electrons within the organic EL element results in efficient electroluminescence.
There have also been proposed three-layer organic EL devices that contain an organic light-emitting layer (LEL) between the hole-transporting layer and electron-transporting layer, such as that disclosed by C. Tang et al. (J. Applied Physics, Vol. 65, 3610 (1989)). The light-emitting layer commonly consists of a host material doped with a guest material, otherwise known as a dopant. Still further, there has been proposed in U.S. Pat. No. 4,769,292 a four-layer EL element comprising a hole injecting layer (HIL), a hole-transporting layer (HTL), a light-emitting layer (LEL) and an electron-transporting/injecting layer (ETL). These structures have resulted in improved device efficiency.
Since these early inventions, further improvements in device materials have resulted in improved performance in attributes such as color, stability, luminance efficiency and manufacturability, e.g., as disclosed in U.S. Pat. Nos. 5,061,569, 5,409,783, 5,554,450, 5,593,788, 5,683,823, 5,908,581, 5,928,802, 6,020,078, and 6,208,077, amongst others.
Notwithstanding these developments, there are continuing needs for organic EL device components, such as light-emitting materials, sometimes referred to as dopants, that will provide high luminance efficiencies combined with high color purity and long lifetimes. In particular, there is a need to be able to adjust the emission wavelength of the light-emitting material for various applications. For example, in addition to the need for blue, green, and red light-emitting materials there is a need for blue-green, yellow and orange light-emitting materials in order to formulate white-light emitting electroluminescent devices. For example, a device can emit white light by emitting a combination of colors, such as blue-green light and red light or a combination of blue light and yellow light.
The preferred spectrum and precise color of a white EL device will depend on the application for which it is intended. For example, if a particular application requires light that is to be perceived as white without subsequent processing that alters the color perceived by a viewer, it is desirable that the light emitted by the EL device have 1931 Commission International d'Eclairage (CIE) chromaticity coordinates, (CIEx, CIEy), of about (0.33, 0.33). For other applications, particularly applications in which the light emitted by the EL device is subjected to further processing that alters its perceived color, it can be satisfactory or even desirable for the light that is emitted by the EL device to be off-white, for example bluish white, greenish white, yellowish white, or reddish white.
White EL devices can be used with color filters in full-color display devices. They can also be used with color filters in other multicolor or functional-color display devices. White EL devices for use in such display devices are easy to manufacture, and they produce reliable white light in each pixel of the displays. Although the OLEDs are referred to as white, they can appear white or off-white, for this application, the CIE coordinates of the light emitted by the OLED are less important than the requirement that the spectral components passed by each of the color filters be present with sufficient intensity in that light. Thus there is a need for new materials that provide high luminance intensity for use in white OLED devices.
A useful class of electron-transporting materials is that derived from metal chelated oxinoid compounds including chelates of oxine itself, also commonly referred to as 8-quinolinol or 8-hydroxyquinoline.
Tris(8-quinolinolato)aluminum (III), also known as Alq or Alq3, and other metal and non-metal oxine chelates, are well known in the art as electron-transporting materials.
Tang et al., in U.S. Pat. No. 4,769,292 and VanSlyke et al., in U.S. Pat. No. 4,539,507 lower the drive voltage of the EL devices by teaching the use of Alq as an electron transport material in the luminescent layer or luminescent zone.
Baldo et al., in U.S. Pat. No. 6,097,147 and Hung et al., in U.S. Pat. No. 6,172,459 teach the use of an organic electron-transporting layer adjacent to the cathode so that when electrons are injected from the cathode into the electron-transporting layer, the electrons traverse both the electron-transporting layer and the light-emitting layer.
The use of a mixed layer of a hole-transporting material and an electron-transporting material in the light-emitting layer is well known. For example, see US 2004/0229081; U.S. Pat. Nos. 6,759,146, 6,759,146; 6,753,098; and 6,713,192 and references cited therein. Kwong and co-workers, US 2002/0074935, describe a mixed layer comprising an organic small molecule hole-transporting material, an organic small molecule electron-transporting material and a phosphorescent dopant.
Tamano et al., in U.S. Pat. No. 6,150,042 teaches use of hole-injecting materials in an organic EL device. Examples of electron-transporting materials useful in the device are given and included therein are mixtures of electron-transporting materials.
Wakimoto and coworkers describe an electroluminescence element including a hole-blocking layer made of an organic compound laminated between the light emitting layer and the electron transport layer. The hole-blocking layer is a mixed layer made of plural kinds of electron transport materials.
Seo et al., in US 2002/0086180 teaches the use of a 1:1 mixture of Bphen, (also known as 4,7-diphenyl-1,10-phenanthroline or bathophenanthroline) as an electron-transporting material, and Alq as an electron injection material, to form an electron-transporting mixed layer. However, the Bphen/Alq mix of Seo et al. shows inferior stability.
Organometallic complexes, such as lithium quinolate have been used in EL devices, for example see WO 0032717 and US 2005/0106412. In particular mixtures of lithium quinolate and Alq have been described as useful, for example see U.S. Pat. No. 6,396,209 and US 2004/0207318.
Commonly assigned U.S. Ser. Nos. 11/076,821 (now abandoned); 11/077,218 (publication No. US 2006/0204784; and Ser. No. 11/116,096 (now abandoned; publication No. US 2006/0246315) describe mixing a first compound with a second compound that is a low voltage electron transport material, to form a layer on the cathode side of the emitting layer in an OLED device, which gives an OLED device that has a drive voltage even lower than that of the device with the low voltage electron transport material. In some cases a metallic material based on a metal having a work function less than 4.2 eV is included in the layer. However, in many cases, incorporating this mixture of compounds in the ETL results in a device whose color is shifted to a less desirable hue.
Thus, previously described devices do not have all the desired EL characteristics. There remains a need to reduce drive voltage of OLED devices while maintaining purity of the color produced.